That thou shalt set apart unto the Lord all that openeth the matrix, and every firstling that cometh of a beast which thou hast; the males shall be the Lord’s. (Exodus 13:12)
This expression is particularly appropriate in the case of firstborn animals, which would have to be separated from the rest of the flock or herd and put aside for the Lord, so as not to be lost among the other lambs, kids, and calves. Since they were not to be sacrificed immediately (Lev. 22:27), it was necessary to keep them separate until disposed of as God commanded.